1. Field of the Invention
This invention relates to a high-magnification objective optical system for use in a binocular stereomicroscope equipped with two observation optical systems for forming light from an object into left and right images through an objective lens.
2. Description of Related Art
Stereomicroscopes are available in Greenough and Galilean (single objective) types. As an example of these types, a single objective type binocular stereomicroscope which is suitable for the purpose of obtaining a relatively high zoom ratio will be explained below.
A fundamental arrangement of an optical system in the single objective type binocular stereomicroscope is shown in FIG. 1. In this figure, the optical system comprises, in order from the side of an object 1, an objective lens 2, a focal zoom lenses 3 and 3', imaging lenses 4 and 4', and eyepieces 5 and 5'. Two optical axes corresponding to the eyes meet on the surface of a specimen, and an angle .theta. made by the two optical axes is called an internal inclination angle. Also, in FIG. 1, reference numerals 6 and 6' represent intermediate image positions. Here, primed and unprimed numerals in each pair indicate identical optical members.
As mentioned above, the single objective type binocular stereomicroscope, unlike an ordinary microscope, has two optical paths for the eyes, symmetrically located in the objective lens 2 with respect to its center, to see the object 1 from left and right sides. The object 1 can thus be viewed stereoscopically as in the case where observation is made with the unaided eye.
Moreover, since the single objective type binocular stereomicroscope is constructed so that the two optical paths are parallel between the objective lens 2 and the afocal zoom lenses 3 and 3' and between the afocal zoom lenses 3 and 3' and the imaging lenses 4 and 4', it is possible to replace the objective lens 2 or interpose, for example, an illumination optical system between the afocal zoom lenses 3 and 3' and the imaging lenses 4 and 4'. Hence, this stereomicroscope has the advantage of accommodating various observations.
The magnification of the objective lens in the single objective type binocular stereomicroscope is represented by the ratio between the focal lengths of the objective lens and the imaging lens. For example, if each of the focal lengths of the objective lens and the imaging lens is 100 mm, the magnification of the objective lens is 1.times.. This magnification is low compared with the case of the ordinary microscope. Specifically, objective lenses for ordinary microscopes range in magnification from 1 to 250.times., whereas those for single objective type binocular stereomicroscopes range from 0.5 to 2.times., and from 0.3 to 15.times. even when variable magnification optical systems are placed therein. In this way, the single objective type binocular stereomicroscope is chiefly used for general work because it has a lower magnification and a smaller numerical aperture (hereinafter referred to as NA) and therefore has a greater depth of focus and longer in working distance.
Since a change of magnification in the single objective type binocular stereomicroscope is mainly carried out by a variable magnification optical system located on the image side of the objective lens, the diameter and position of the exit pupil of the objective lens are changed, and the NA and a real field of view are also changed. In the case of a 1.times. objective lens, the NA and the real field of view undergo changes in the range from about 0.01 and 32 mm in diameter to about 0.1 and 6 mm in diameter, respectively. Such changes are illustrated in FIGS. 2A and 2B. FIG. 2A shows the case where the NA is high and the real field of view is small, and FIG. 2B shows the case where the NA is lower and the real field of view is larger. Also, in these figures, reference numeral 7 denotes the surface of a specimen (an object to be observed), 8 denotes an objective lens, and 9 denotes the exit pupil of the objective lens 8.
As stated above, in the single objective type binocular stereomicroscope, the change of magnification is often made by the variable magnification optical system. Where the specimen is observed at a higher or lower magnification, it is only necessary to replace the objective lens because of the structural feature of the single objective type binocular stereomicroscope. By doing so, in an ordinary case, magnifications of 0.75.times. and 0.5.times. are obtained at the low magnification side and magnifications of 1.5.times. and 2.times. at the high magnification side.
Although, in the above description, the single objective type binocular stereomicroscope has been specifically explained by way of example, a Greenough type stereomicroscope may be thought of as substantially the same as the single objective type binocular stereomicroscope with the exception that objective lenses are separately placed for the eyes.
In recent years, demands on observations with high magnification and NA have increased to make an assembly and profile inspection of electronic circuit parts in the industrial field and to sort out medium cells and oval cells in the field of biology. Thus, in the stereomicroscope, various attempts are made to bring the high magnification and NA to the variable magnification optical system and the objective lens. For the binocular stereomicroscope characterized by stereoscopic observation, however, it is necessary to solve the following problems in order to obtain the high magnification and NA.
Since most of specimens used in the industrial field are usually opaque to light, it is common practice to perform observations with a reflecting illumination system. In this case, light emitted from a light source passes through an illumination system, a variable magnification optical system, and an objective lens to irradiate a specimen. As a result, it is known that the light is reflected by lens surfaces of the variable magnification optical system and the objective lens, and thus deterioration in contrast is brought about. Hence, in coaxial reflecting illumination, a means such as that shown in FIG. 3 has been used to prevent the deterioration of contrast.
Specifically, light emitted from light sources 10 and 10' passes through polarizers 11 and 11' to change into linearly polarized light, which is reflected by half mirrors (or prisms) 12 and 12' and travels through afocal zoom lenses (variable magnification optical systems) 13 and 13' and an objective lens 14. Since a quarter-wave plate (or a .lambda./4 plate) 15 is placed in front of the objective lens 14, the light passing through the quarter-wave plate 15 is converted into circularly polarized light to irradiate a specimen surface 16. The circularly polarized light, when reflected by the specimen surface 16, rotates in a reverse direction. Thus, when this reflected light passes through the quarter-wave plate 15 again, it rotates by 90.degree., compared with the linearly polarized light which is initially incident. The quarter-wave plate 15 is transparent to only the light from the specimen surface 16 to prevent a reduction in contrast. In this way, the light having passed through the quarter-wave plate 15 is introduced through the afocal zoom lenses 13 and 13' and analyzers 17 and 17' into eyepieces 18 and 18'.
Thus, in the case of the coaxial reflecting illumination, a quarter-wave plate with a thickness of about 4-6 mm is placed in front of the objective lens, and thereby the deterioration of contrast has been prevented. Even though the magnification and the NA are increased by the variable magnification optical system, the placement of the quarter-wave plate does not so adversely affect the performance of image formation if the magnification of the objective lens is approximately 1.times.. Where the magnification is further increased to make observation, however, as mentioned above, it is necessary to replace the objective lens with that of higher magnification. In this way, if the magnification and the NA are increased, aberration attributable to the thickness of the quarter-wave plate will be produced. This gives rise to the problem of degrading the performance of image formation. Consequently, where the quarter-wave plate is used and observation is made by means of the coaxial reflecting illumination, the magnification of the objective lens has been limited to 1.5.times..
On the other hand, specimens used in the field of biology are sometimes observed through water in vessels. The thickness (depth) of water in this case is usually about 1-10 mm. When observation is made under such circumstances, little problem arises in observation with low magnification. However, when observation is made at high magnification and NA in order to view an intracellular structure of a very small object such as an oval cell, the performance of image formation is degraded due to the thickness of water, and an image to be observed is blurred in spite of the fact that the magnification is increased by the objective lens of high magnification. This causes the problem that desired performance is not obtained.
Furthermore, the objective lens for binocular stereomicroscopes is designed so that axes of two optical paths lying at a predetermined distance apart are obliquely directed toward the specimen for image formation. As such, the single objective type binocular stereomicroscope has the following defects because two optical axes in the objective lens are located away from the center of the objective lens. FIGS. 4A, 4B, and 4C show cases where objective lenses have different magnifications of 0.5.times., 1.times., and 2.times., respectively. As seen from these figures, the internal inclination angle .theta. increases with increasing magnification of an objective lens 20, and the optical axes between a specimen surface 19 and the objective lens 20 are inclined increasingly. Thus, there is the problem that as the magnification of the objective lens 20 is increased, the production of aberration becomes pronounced. Also, in these figures, reference numerals 21 and 21' represent afocal zoom lenses (variable magnification optical systems) and 22 and 22' represent eyepieces.
As has been mentioned, when the objective lens for binocular stereomicroscopes is designed to have a high magnification, various problems are raised. Since, in particular, the conventional stereomicroscope has been designed on the premise that only air is interposed between the specimen and the objective lens, it is an unquestionable fact that if media of different refractive indices are interposed therebetween, a resolving power cannot be improved for observation even when the objective lens of high magnification is used.